MgdSchema needs required fields

[bergie]

To synchronize MgdSchema's features a bit with JCR Node Type Notation, we should support required properties.

If object is being saved without a required field having a value, Midgard should throw an exception and abort that save.

Something like:

<type name="org_openpsa_products_product">
    ...
    <property name="title" type="string" required="yes"/>
</type>

$product = new org_openpsa_products_product();
$product->description = "Blah";
// No title set
$product->create(); // Will throw midgard_error_requiredfield exception and not save the product

(copied from http://trac.midgard-project.org/ticket/761)